The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting an end-point of a metal layer during a chemical mechanical polishing operation.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate may be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
Several methods have been developed for in-situ polishing endpoint detection. Most of these methods involve monitoring a parameter associated with the substrate surface, and indicating an endpoint when the parameter abruptly changes. For example, where an insulative or dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed.
Where the monitored parameter changes abruptly at the polishing endpoint, such endpoint detection methods are acceptable. However, as the substrate is being polished, the polishing pad condition and the slurry composition at the pad-substrate interface may change. Such changes may mask the exposure of an underlying layer, or they may imitate an endpoint condition. Additionally, such endpoint detection methods will not work if only planarization is being performed, if the underlying layer is to be over-polished, or if the underlying layer and the overlying layer have similar physical properties.
In one aspect, the invention is directed to a method of determining polishing parameters. In the method, a surface of a substrate is brought into contact with a polishing pad that has a window, relative motion is created between the substrate and the polishing pad, and a light beam is directed through the window. The motion of the polishing pad relative to the substrate causes the light beam to move in a path across the substrate. Light beam reflections from a layer in the substrate are detected, reflection data associated with the light beam reflections is generated, and the reflection data from a scan of the light beam across the substrate is displayed. Polishing parameters are selected to provide uniform polishing of the substrate based on the displayed reflection data.
Implemenations of the invention may include one or more of the following features. The displayed reflection data may show the reflectivity of the substrate as the light beam scans across the substrate. The reflectivity of the substrate may be displayed in real-time during polishing. The layer maybe a metal. The reflection data may include a plurality of intensity measurements made at a plurality of positions along the path across the substrate. A radial position relative to the center of the substrate may be calculated for each intensity measurement. The reflection data may be divided into a plurality of radial ranges, and which radial range is the last portion to be completely polished may be determined. The displayed reflection data may form at least one transient signal graph. Each transient signal graph may comprise reflection data from a single sweep of the window beneath the substrate.
In another aspect, the invention is directed to a method of generating endpoint parameters. A first substrate is polished, light beam reflections are detected during polishing the first substrate to generate a first plurality of intensity measurements, and a radial range to use for endpoint detection is determined from the first plurality of intensity measurements. A second substrate is polished, light beam reflections are detected during polishing of a layer in a second substrate to generate a second plurality of intensity measurements, a radial position relative to the center of the substrate is calculated for each of the second intensity measurements, and a polishing endpoint is determined from those second intensity measurements which are within the radial range.
Implementations of the invention may include one or more of the following features. Determining the radial range may include determining the last portion of the substrate to be completely polished. At least one process parameter may be determined for polishing of the second substrate from the first plurality of intensity measurements.
In another aspect, the invention relates to a method of determining process uniformity. In the method, light beam reflections are detected during polishing of a layer in first and second substrates. Reflection data associated with the light beam reflections is generated, and the reflection data is displayed from a first scan of the light beam across the first substrate and from a second scan of the light beam across the second substrate. The reflection data from the first scan is compared to the reflection data from the second scan to determine process uniformity. A polishing consumable may be changed between the polishing of the first and second substrates.
Advantages of the invention include one or more of the following. The reflection data from a wafer is captured using a high resolution data acquisition system at a relatively fine time scale, on the order of milliseconds. Further, reflection intensity changes during polishing are captured for different radial positions on the substrate. The high resolution data acquisition system provides precise time control of each process step in a multi-step operation. Detailed data is available on the progress of the metal polishing operation at different locations of the wafer. Additionally, parameters such as uniformity of the entire wafer and removal rate for different radial portions of the wafer are determined. The acquired high resolution data can be processed on-line or off-line to adjust various variables and parameters to minimize erosion and dishing of the surface layer. If the data is processed in real-time, the feedback data may be used for endpoint detection or for closed-loop control of process parameters. For instance, the polishing pressure, polishing speed, chemistry, and slurry composition may be altered in response to the feedback data to optimize the overall polishing performance and/or polishing quality. The reflection data is available for experimentation to improve the deposition process.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.